New power systems operating on fossil fuels have been under development for several years. These systems are designed to increase efficiency (fuel energy conversion to electricity) and to reduce harmful emissions (NOx, CO, CO2) to the environment. Cogeneration and combined cycle system approaches can increase the efficiency by more than 20% compared to conventional power systems.
Several cogeneration and combined-cycle power systems of various configurations have been proposed that have the potential for achieving relatively high efficiencies. However, these systems depend on obtaining solutions to certain technical problems related to the concept. For example, these systems do not minimize harmful pollutants or maximize thermodynamic efficiency because they do not provide for recovery of synthesis from CO2, and do not use fuel that passes through the fuel cells unreacted and do not efficiently use “waste heat” generated be the fuel cell stack.
Regarding thermodynamic inefficiency, the waste heat energy generated by fuel cells in these systems is used to drive closed water or open air power cycles. Closed water or open air power cycles can be thermodynamically modeled as reversible heat cycles, if losses such as frictional losses are ignored. For a reversible heat cycle which operates between two temperatures, maximum TH and minimum TC, the maximum cycle efficiency (e) is limited by the Carnot relation/equation e=1−(TC/TH), where both temperatures are expressed in units of Kelvin.
Thus, the maximum theoretical efficiency of a closed water or open air power cycle is maximized when the cold reservoir is held as cold as possible, and the hot reservoir is held as hot as possible. Consequently, since the range of attainable practical high and low temperatures are limited, the maximum possible efficiency derivable from these reversible heat cycles are lower than the Carnot limit. As a result, practical efficiencies of these closed water or open air cycles cannot be higher than approximately 30 to 35%. Thus, the total efficiency of the overall process of energy conversion to electricity for an entire combined cycle cannot exceed approximately 55 to 60%. To further maximize efficiency of combined cycle power systems which use fuel cells, a new combined cycle system is needed.